The present invention relates to a glass composition having a high rigidity and a high modulus and a process for producing the same. More particularly, this invention relates to a glass composition suitable for use as a substrate for information recording media which is required to be of high quality and have excellent surface smoothness and a high modulus of elasticity, and to a process for producing the glass composition. The present invention further relates to a substrate for information recording media and an information recording medium both comprising the glass composition.
Information recording devices such as magnetic disks are always required to have a larger recording capacity and to attain a reduction in access time such as disk rotational delay. One possible means for satisfying the latter requirement is to heighten the rotational speed of a medium.
However, the substrates or media currently in use are weighed down by themselves and resonate considerably at an increased rotational speed. Eventually, the surface of such a medium comes into contact with the head to cause an error or crushing. It is therefore impossible to narrow the gap between the magnetic disk head and the recording medium to or below a certain level, and this constitutes a serious obstacle to an increase in recording capacity.
For reducing the bending of a substrate or medium and diminishing the resonance of the medium being rotated, it is necessary to heighten both the modulus of elasticity (Young""s modulus) of the substrate and the rigidity thereof which is the value obtained by dividing the modulus of elasticity by the specific gravity. However, the aluminum alloy which has been most commonly used as the substrates of magnetic disks has a modulus of elasticity of 71 GPa and a rigidity of 26 GPaxc2x7cm3/g. This conventional substrate material, having such properties, hardly copes with the trend toward higher rotational speeds of 10,000 rpm and above. In addition, it has become necessary to increase the thickness of substrates made of the above material, although this goes against the current trend toward thickness reduction in disk substrates for device miniaturization.
In contrast, substrates made of a chemically strengthened glass are superior to the aluminum substrate in both modulus of elasticity and specific gravity.
For example, a glass substrate obtained by subjecting a commercial soda-lime glass to ion exchange in a molten potassium salt is on the market. This substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPaxc2x7cm3/g.
Also known besides the above one is a glass substrate obtained by chemically strengthening commercial Corning 0317. Although this substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPaxc2x7cm3/g, these properties are still insufficient.
High-rigidity substrates for information recording media which are made of a material other than chemically strengthened glasses are on the market. These substrates comprise a crystallized glass having a modulus of elasticity of 90 GPa and a rigidity of 38 GPaxc2x7cm3/g. However, these substrates, after polishing, inevitably have residual crystal grains projecting from the surface because of the nature of the production process in which crystals are precipitated inside. Namely, these crystallized-glass substrates have a drawback that they are inferior in surface smoothness to the substrates made of a chemically strengthened glass.
Consequently, in view of the expected future trend toward even higher rotational speeds in information recording devices, there is a desire for a glass composition which has further improved properties, i.e., which has a high Young""s modulus and a high rigidity, can be easily strengthened chemically, and gives a substrate having high surface smoothness through polishing.
One object of the present invention is to provide a glass composition which has a high value of modulus of elasticity represented by Young""s modulus and a high value of rigidity defined by (Young""s modulus)/(specific gravity) and is capable of being effectively inhibited from resonantly vibrating or reduced in the amplitude of resonant vibration.
Another object of the present invention is to provide a glass substrate for information recording media which comprises the glass composition.
Still another object of the present invention is to improve glass quality by diminishing bubbling during glass article production.
The present invention has been achieved in view of the above-described problems of prior art techniques and the above-described requirements.
The present invention provides a glass composition comprising the following basic components in terms of mol %: 40 to 65% SiO2, 5 to 25% Al2O3, 2 to 20% Li2O, 0 to 9% Na2O, 0 to 10% TiO2, 0 to 10% ZrO2, 0 to 25% MgO, 0 to 25% CaO, and 0 to 10% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from 2 to 40% and the sum of (Li2O)/2 and Na2O is from 1 to 10 mol %, said glass composition further containing from 0.01 to 5 mol % SnO2 and up to 0.1 mol % sulfur (S) in terms of the amount of SO3.
The glass composition preferably has a rigidity as defined by (Young""s modulus)/(specific gravity) of 30 GPaxc2x7gxe2x88x921xc2x7cm3 or higher and a modulus of elasticity as represented by Young""s modulus of 90 GPaxc2x7or higher.
Furthermore, the glass composition is preferably one which has undergone an ion exchange treatment in at least one molten salt containing ions of potassium, sodium, or both.
The present invention further provides a process for producing a glass composition comprising the following basic components in terms of mol %: 40 to 65% SiO2, 5 to 25% Al2O3, 2 to 20% Li2O, 0 to 9% Na2O, 0 to 10% TiO2, 0 to 10% ZrO2, 0 to 25% MgO, 0 to 25% CaO, and 0 to 10% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from 2 to 40%, wherein SnO2 and sulfur are further added to the composition, so that the glass composition further contains, as clarifiers, from 0.01 to 5 mol % SnO2 and up to 0.1 mol % sulfur (S) in terms of the amount of SO3 when the sum of (Li2O)/2 and Na2O is from 1 to 10 mol %.
In the process for producing a glass composition, a sulfuric acid salt compound is preferably used as part of batch materials for the composition.
Furthermore, in the process for producing a glass composition, the glass composition is preferably molded by a float process.
The present invention still further provides a substrate for information recording media which comprises the above-described glass composition which has undergone the ion exchange treatment. An information recording medium containing the substrate is also provided by the invention.
The present inventors made intensive investigations on glass compositions having a high Young""s modulus. As a result, they have found that it is necessary to limit the contents of Na2O and K2O, which are alkali metal oxides, to or below certain levels. In particular, since K2O is highly effective in lowering Young""s modulus, the content thereof is preferably not higher than 0.1 mol %, which is an allowable limit of the amount of K2O which can come into a glass as an impurity. More preferably, the content of K2O is substantially zero.
Na2O is the most effective after K2O in lowering Young""s modulus. The content thereof is hence preferably 9 mol % or lower, more preferably 5 mol % or lower.
In producing soda-lime glass compositions to be generally molded by a float process, a sulfuric acid salt compound, especially Na2SO4, is used as a clarifier. The Na2SO4 decomposes in the course of glass melting, and part of the sulfur dissolves as SO2, SO3, and other forms in the glass. The dissolved sulfur ingredients absorb bubbles in the course of glass cooling, whereby a bubble-free glass of high quality can be produced.
It has however been found that glasses having a low content of alkali metal oxides, especially Na2O and Li2O, have a reduced content of residual sulfur.
The inventors have further found that when the residual sulfur content in a glass is expressed in terms of the amount of SO3, this amount correlates with the content of Li2O and Na2O in the glass, i.e., with (Li2O)/2+Na2O (mol %). For example, soda-lime glasses in which the amount of (Li2O) /2+Na2O has been reduced to 10 mol % or 5 mol % have an amount of residual SO3 about one-half or about one-fifth, respectively, the amount of residual SO3 in ordinary soda-lime glasses.
There has hence been a problem that in glasses having a low residual SO3 content, a sufficient clarifying effect cannot be obtained with Na2SO4, which is an ordinary clarifier.
The present inventors furthermore made investigations on a method for glass composition clarification applicable also to a float process, which is a molding technique capable of producing especially highly flat glass plates.
As a result of intensive studies made by the present inventors in order to eliminate the above-described problem, they have found that a sufficient clarifying effect is obtained in a glass in which the amount of (Li2O)/2+Na2O is 10 mol % or smaller and which contains SnO2 in an amount of from 0.01 to 5 mol %. The presence of SnO2 is effective especially in a glass in which the amount of (Li2O)/2+Na2O is 5 mol % or smaller.
Usable materials for the SnO2 include SnO2, SnO, and other tin compounds.
It has also been found that the above clarifying effect is further enhanced by using a sulfuric acid salt compound as one of the batch materials for the above glass. The present invention has been thus achieved.
Examples of the sulfuric acid salt used as a batch material include Li2SO4, Na2SO4, MgSO4, CaSO4, SrSO4, and BaSO4. Especially preferred of these are Li2SO4 and Na2SO4. If the sulfur content in the batch materials is too low, the clarifying effect thereof is insufficient. Even if the content thereof is too high, the clarifying effect thereof is not heightened any more and the result is increased emission of SOx gases causative of air pollution. Consequently, the sulfuric acid salt as a batch material is used in an amount of generally from 0.1 to 10%, preferably from 0.2 to 2%, in terms of sulfur amount based on the SiO2 (mol %) contained in the glass to be produced. When the sulfuric acid salt is used in a proportion within the above range, the glass obtained has a residual sulfur content of from 0 to 0.1 mol %.
Although As2O3, Sb2O3, and compounds thereof have hitherto been generally employed as clarifiers, these compounds are undesirable because of their toxicity. In the case of using a float process, use of As2O3 or Sb2O3 is more undesirable in that it is reduced by tin metal to generate defects on the glass surface.
The reasons for limitations of the makeup of the glass composition are as follows. Hereinafter, all percents are by mole.
SiO2 is the main component constituting the glass. If the proportion of SiO2 is lower than 40%, the glass has impaired chemical durability. On the other hand, if the proportion thereof exceeds 64%, the desired modulus of elasticity and rigidity are not obtained. Consequently, the proportion of SiO2 should be from 40 to 65%.
Al2O3 is an ingredient which improves the rigidity of the glass and increases the depth of a compression stress layer formed by ion exchange. Al2O3 further serves to improve the water resistance of the glass. If the proportion of Al2O3 is lower than 5%, these effects are insufficient. On the other hand, if the proportion thereof exceeds 25%, the results are an increased viscosity, an increase in liquidus temperature which is severer than the viscosity increase, and impaired meltability. Consequently, the proportion of Al2O3 should be from 5 to 25%, and is preferably from 10 to 20%.
Li2O, which is an ingredient to be replaced in ion exchange, serves to lower the melting temperature of the glass to thereby enhance its meltability. If the proportion of Li2O is lower than 2%, ion exchange cannot be conducted and the glass has too high a melting temperature. On the other hand, if the proportion thereof exceeds 20%, the substrate has impaired weatherability and impaired acid resistance. Consequently, the proportion of Li2O should be from 2 to 20%, and is preferably from 2 to 15%.
Na2O, which is an ingredient to be replaced in ion exchange, serves to lower the melting temperature and the liquidus temperature to thereby enhance meltability. If the proportion of Na2O exceeds 9%, the glass has a reduced modulus of elasticity and impaired weatherability and acid resistance. Consequently, the proportion of Na2O should be 9% or lower, and is preferably 5% or lower.
TiO2 is an ingredient which improves the modulus of elasticity and weatherability of the glass. However, if the proportion thereof exceeds 10%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of TiO2 should be 10% or lower.
ZrO2 is an ingredient which improves the modulus of elasticity and weatherability of the glass. However, if the proportion of ZrO2exceeds 10%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of ZrO2 should be 10% or lower.
MgO is an ingredient which heightens the modulus of elasticity and meltability of the glass. However, if the proportion of MgO exceeds 25%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of MgO should be 25% or lower.
CaO is an ingredient which heightens the modulus of elasticity and meltability of the glass. However, if the proportion of CaO exceeds 25%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of CaO is preferably 25% or lower.
SrO is an ingredient which enhances the meltability of the glass. However, if the glass contains SrO in a large amount, it disadvantageously has an increased specific gravity and a reduced rigidity. Consequently, the proportion of SrO is preferably 10% or lower.
If the total amount of MgO, CaO, and SrO (i.e., the amount of RO) is below 2%, the glass has a low modulus of elasticity and insufficient meltability If the total amount thereof exceeds 40%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the total amount of RO is preferably from 2 to 40%.
SnO2 is an ingredient which not only improves the modulus of elasticity and weatherability of the glass, but also serves as an effective clarifier. If the proportion of SnO2 is lower than 0.01%, sufficient effects cannot be obtained. On the other hand, proportions thereof exceeding 5% are undesirable in that devitrification occurs to generate SnO2 crystals. Consequently, the proportion of SnO2 is generally from 0.01 to 5%, preferably from 0.1 to 2%.
Besides the ingredients described above, other ingredients may be added in a total amount of up to 3%. Examples of such optional ingredients include Fe2O3, CoO, NiO, and MnO for coloring, and further include ZnO, Y2O3, La2O3, and CeO2.
This glass, which contains Li2O or contains Li2O and Na2O, can be made to have an increased fracture strength by immersing the glass in at least one molten salt containing ions of potassium and sodium or of either at a temperature not higher than the distortion point of the glass to thereby interchange these ions and thus generate a compression stress on the surface of the glass.
When this glass composition is used as a substrate for information recording media, this substrate is less apt to bend or suffer resonant vibration because it has a high rigidity. Therefore, the recording medium employing this glass composition is especially suitable for use in recording apparatuses of the high rotational speed type.
Since the glass composition described above contains not As2O3, nor Sb2O3, which are toxic, but SnO2 as a clarifier, it is excellent in quality with respect to freedom from bubbles. The glass composition is preferred also in that due to the nonuse of As2O3 or Sb2O3, it develops no surface defects even when molded by a float process.